1. Field of the Invention
The invention relates generally to vehicle steering systems. More particularly, the invention relates to a vehicle steering system which includes right and left steering operation mechanisms to respectively steer right and left steered wheels, and in which a steering member to be operated in steering a vehicle in a desired direction is mechanically coupled to neither of the right and left steering operation mechanisms, and the right and left steering operation mechanisms are respectively driven by right and left steering motors.
2. Description of the Related Art
The effectiveness of a “steer-by-wire” system is now being valued highly. A steer-by-wire system implements an advanced drive assisting function as typified by automatic driving and has no intermediate shaft so as to increase layout flexibility of an engine room. To further increase engine room layout flexibility, Japanese Patent Application Publication No. 2008-174160 (JP 2008-174160 A) and Japanese Patent Application Publication No. 2015-20586 (JP 2015-20586 A) each disclose a right and left independent steering system for controlling right and left steered wheels by individual steering actuators without using any steering gear device including a rack-and-pinion mechanism.
A steer-by-wire system that employs the concept of the right and left independent steering system may perform steering control using a control system including a higher-level control device to calculate target steered angles for right and left steered wheels, and a lower-level control device to control steering actuators. The lower-level control device of such a control system receives the target steered angles for the right and left steered wheels from the higher-level control device, and controls the steering actuators on the basis of the target steered angles associated thereto. Unfortunately, a malfunction in the higher-level control device of such a control system, such as a breakdown or communication failure, may cause the higher-level control device to supply an inappropriate target steered angle to the lower-level control device.